1. Cross-references
This application contains subject matter generic to the temperature responsive control disclosed in copending application Ser. No. 428,187 filed Nov. 29, 1974.
2. Field of the Invention
This invention relates generally to speed control of fluid couplings and specifically to speed control in response to temperature and speed.
3. Description of the Prior Art
It is well known to regulate the speed of an engine driven cooling fan in accordance with cooling demand, rather than in accordance with engine speed. The cooling demand is low during engine warm up, cold weather operation, and high vehicle speed. By contrast, demand is high during hot weather, particularly when the vehicle is not moving and the engine is idling.
The use of fluid couplings of the viscous shear type and of the hydrokinetic type to drive engine cooling fans for vehicles is well known. The output speed of such couplings may be controlled in accordance with cooling requirements, thereby allowing lower fan speeds when cooling requirements are low; this has the advantage of reducing engine fuel consumption and fan noise.
Controls to vary the output speed of such couplings in response to temperature and/or speed are well known in the prior art. Temperature control has been provided by a bimetallic spring in many of the viscous coupling controls. In such controls the spring rotates with the coupling and applies a force to move a valving member by bending or straightening in response to temperature changes. However, such springs generate a relatively small force per degree temperature change. Therefore, the controls are susceptible to erratic operation due to rotational forces acting on the spring and/or moveable parts of the control. Further, foreign matter in the control or corrosion of the moveable parts can easily render the control totally inoperative.
If a viscous coupling is to be controlled in response to ambient temperature exterior of the coupling, the spring (by necessity) is positioned on the coupling exterior. When exteriorly positioned, the spring must also move an intermediate mechanism which passes through a wall of the coupling. The force needed to reliably move the intermediate mechanism and the valving member is close to the maximum practical force which can be generated by the spring; therefore, the spring must be located on the coupling in a position least influenced by rotational forces. Exteriorly positioned springs are traditionally positioned coincident the rotational axis of the coupling to avoid rotational forces. However, this is a poor position for a temperature sensing element, since better temperature responses can be obtained at a radial extremity of the coupling where air flow is greater.
Erratic operation of the control due to sticking may be further compounded when a bimetallic spring is used in combination with a centrifugal weight for limiting maximum speed of the coupling, since the forces produced by the weight may be low so that they are in proportion to the low force produced by the spring.